It is well known to disperse an electrically conductive material in an electrically insulating polymer to prevent static charge or other purposes and obtain an electrically conductive polymer (see, for example, Japanese Patent Publication (Kokoku) No. 58-39175). As electrically conductive materials which are admixed with polymers, ionic or nonionic organic surfactants, metal powders, electrically conductive metal oxide powders, carbon black, carbon fibers, and the like are generally used. There are dispersed in a polymer by melting and kneading to form an electrically conductive polymer composition, which is shaped to obtain an electrically conductive article having a volume resistivity of 10.sup.0 -10.sup.10 .OMEGA..multidot.cm.
It is also known that use of a material having a large aspect ratio such as flakes or whiskers as the conductive material can provide a polymer with electrical conductivity using a relatively small amount. This is because a conductive material having a large aspect ratio increases the number of contact points between the material for the same unit weight, so it is possible to obtain electrical conductivity using a smaller amount.
However, a conventional electrically conductive polymer composition has problems with respect to stability at high temperatures (heat resistance and dimensional stability), moldability, and color.
For example, when an organic surfactant is used as the conductive material, the heat resistance is poor, and the electrical conductivity is easily influenced by humidity. An inorganic conductive material is usually in the form of spherical particles, so it is necessary to mix a large quantity exceeding 50 wt % based on the total weight of the composition, so the physical properties of the polymer worsen, and its moldability into filaments or films is decreased.
Even with flake-shaped or whisker-shaped conductive materials having a large aspect ratio, it has been conventionally necessary to use them in an amount exceeding 40 wt % based on the total weight of the composition. When such a large amount of an electrically conductive material is mixed in a polymer, a directionality (anisotropy) develops at the time of shaping, and the moldability and electrical conductivity are worsened.
In the case of carbon black, if the amount required to impart electrical conductivity (generally at least 10 wt % based on the total weight of the composition) is used, the composition becomes black, and a white or colored formed product can not be obtained.
Carbon fibers, and particularly graphitized carbon fibers, have good electrical conductivity, and it has been attempted to disperse carbon fibers into a polymer as a conductive material. In particular, carbon fibers formed by vapor phase growth method (pyrolysis method) and graphitized, if necessary, by heat treatment, and which are hollow or solid with a fiber diameter of from 0.1 .mu.m to several .mu.m have high electrical conductivity and have attracted attention as a conductive material. However, even with such carbon fibers, when they are admixed in an amount sufficient to impart electrical conductivity, the polymer composition ends up becoming black.
Recently, carbon microfibers with a far smaller fiber diameter than carbon fibers formed by the vapor phase growth method (referred to below as hollow carbon microfibers) have been developed. See, for example, Japanese Patent Publications (Kokoku) Nos. 3-64606 and 3-77288, Japanese Patent Laid-Open (Kokai) Applications Nos. 3-287821 and 5-125619, and U.S. Pat. No. 4,663,220. These microfibers have an outer diameter of less than 0.1 .mu.m, and normally on the order of several nanometers to several tens of nanometers. As they have a slenderness of the nanometer order, they are also referred to as nanotubes or carbon fibrils. They are usually extremely fine hollow carbon fibers having a tubular wall formed by stacking of layers of graphitized carbon atoms in a regular arrangement. These hollow carbon microfibers are used as a reinforcing material in the manufacture of composite materials, and it has been proposed to mix them into various types of resins and rubber as a conductive material. (See, for example, Japanese Patent Laid-Open (Kokai) Applications Nos. 2-232244, 2-235945, 2-276839, and 3-55709).
In Japanese Patent Laid-Open (Kokai) Application No. 3-74465, a resin composition is disclosed which is imparted electrical conductivity and/or a jet black color and which is formed from 0.1-50 parts by weight of carbon fibrils (hollow carbon microfibers) in which at least 50 wt % of the fibers are intertwined to form an aggregate, and 99.9-50 parts by weight of a synthetic resin. In that application, it is described that it is preferred to use at least 2 parts by weight of hollow carbon microfibers to impart electrical conductivity, and when imparting only a jet black color, the amount used is preferably 0.1-5 parts by weight.
As described above, carbonaceous conductive materials have excellent heat stability and can impart electrical conductivity to a polymer by using in a relatively small amount, but they have the drawback that they end up blackening the polymer. Uses for conductive polymers include antistatic mats, electromagnetic wave shield materials, IC trays, building materials, and packaging for film, and in each of these uses, there is a strong need to be able to freely perform coloring, either for reasons of visual design or to permit differentiation of products (such as in the case of IC trays).
An object of the present invention is to provide an electrically conductive polymer composition which has excellent electrical conductivity, heat resistance, and moldability, and which can be used to form a white or colored product by any melt-molding method including melt spinning, melt extrusion, and injection molding.
A more specific object of the present invention is to provide a white or freely colored electrically conductive polymer composition which uses a carbonaceous conductive material and which can be used to form a product of a desired color.